Vulnerability to alcohol dependence is heritable, with a rate ranging from 0.52 to 0.64 (Kendler, 2001). Despite this high heritability rate, only one marker allele (alcohol-metabolizing aldehyde dehydrogenase genes) has been identified consistently to be associated with alcoholism (Kranzler et al, 2002). Of the various neurotransmitter systems through which alcohol mediates its effects, the serotonergic system has been shown to play an important role in alcohol preference and consumption (Johnson, 2004). Synaptic serotonergic neurotransmission is terminated when serotonin (5-HT) is transported back into pre-synaptic neurons by 5-HT transporters (5-HTTs) (Talvenheimo and Rudnick, 1980). Therefore, a major part of the functional capacity of the serotonergic system is regulated by the 5-HTT. Heavy episodic drinking is associated with numerous psychiatric and general medical conditions causing a major public health burden (Cargiulo, 2007). Several studies have reported a dose-response relationship between the extent of heavy drinking and the risk of alcohol related morbidity and mortality among heavy drinkers (Makela and Mustonen, 2007; Gastfriend et al., 2007). Consequently, reduction of heavy drinking is used as an indicator of treatment response in clinical trials aimed at treating alcohol dependence.
Of the various neurotransmitter systems through which alcohol mediates its effects, the serotonergic system has been shown to play an important role in alcohol preference and consumption (Johnson, 2004). Synaptic serotonergic neurotransmission is terminated when serotonin (5-HT) is transported back into pre-synaptic neurons by 5-HT transporters (5-HTTs) (Talvenheimo and Rudnick, 1980) and the degree of 5-HT reuptake depends on the density of 5-HTTs on presynaptic surface. The selective 5-HT reuptake inhibitors that act directly on 5-HTTs have been shown to reduce alcohol consumption in rats (Gill and Amit, 1989). However, in humans SSRIs have been effective at reducing heavy drinking only among some subtypes of alcoholics, more specifically in type A alcoholics but not in type B alcoholics (Dundon et al., 2004; Pettinati et al., 2000) who are considered to be more biologically predisposed to develop alcohol dependence. Therefore, it is reasonable to propose that allelic variations which alter expression levels of SLC6A4 gene can be expected to have an important effect on drinking intensity.
The human 5-HTT is encoded by a single gene (SLC6A4) mapped on chromosome 17q11.1-q12 (Ramamoorthy et al., 1993). The SLC6A4 gene spans ˜35 kb and has 14 exons. The protein encoded by this gene, the 5-HTT, is a trans-membrane protein containing 630 amino acids (Heils et al., 1996). The expression level of SLC6A4 is regulated by at least three mechanisms: transcription regulatory elements in the promoter (Ramamoorthy et al., 1993), differential splicing (Bradley and Blakely, 1997), and the use of different 3′ polyadenylation sites (Battersby et al., 1999). Furthermore, several other polymorphisms that change amino acid sequence (Thr4Ala, Gly56Ala, Glu215Lys, Lys605Asn and Pro612Ser) of 5-HTT have been shown to affect 5-HT uptake function in cell cultures (Prasad et al., 2003).
Although the long (L) and short (S) polymorphism at 5-HTT linked polymorphic region (5-HTTLPR) of SLC6A4 has been extensively studied in the literature, the results are inconclusive. For example, in a meta-analysis of 17 studies, Feinn et al. (2005) showed that S allele was significantly associated with alcohol dependence in subjects with co-occurring serotonergic abnormalities while several other studies reported an association of alcohol dependence with the L allele (Kweon et al., 2005, Hu et al., 2005). On the other hand, numerous studies including the report by our group reveal a differential association between chronic problem-drinking and the density and function of serotonin transporters in alcoholic subjects carrying L and S variants of SLC6A4 (Little et al., 1998, Javors et al., 2005, Johnson et al., 2008). Located in the gene's transcriptional control region, 5-HTTLPR contains 16 tandem repeats of a 20 to 23 bp (G+C)-rich sequence between by −1376 and bp-1027. Two common forms of this transcriptional control region have been found: a long 528 bp allele (L) with 16 repeats and a short 484 bp allele (S) with a deletion of
44 bp extending from bp −1255 to bp −1212.
Serotonin (5-HT) function has been implicated in the regulation of mood, impulsivity, and alcohol use that includes variation in the age of onset of drinking and onset of alcohol use disorders. The 5-HT system, originating in the raphe nuclei and projecting to cortex, hippocampus, and subcortical brain regions, is thought to influence drinking behavior directly in alcohol-use-disordered individuals by modulating the reinforcing effects of alcohol and/or indirectly by processes regulating impulsivity and affect. Findings from animal studies have shown that pharmacological enhancement of 5-HT activity inhibits alcohol intake. Human studies have shown that low 5-HT turnover is associated with impulsivity], as well as alcohol-seeking behavior and alcoholism. Lower central 5-HT turnover (e.g., 5-hydroxy indole acetic acid in cerebrospinal fluid) has been reported in early-onset alcohol-dependent (EOA) adults compared to late-onset alcohol dependent adults (LOA) and the lowest central 5-HT turnover occurs in EOA adults when both parents have alcohol dependence. Together, these findings support the hypothesis that 5-HT availability and function regulate drinking-related behaviors and drinking history.
Scientific frustration has been promulgated by failures to demonstrate clinical efficacy for selective serotonin reuptake inhibitors (SSRIs) in treating alcoholism. Animal studies show consistently that SSRIs reduce alcohol consumption in various models and across species (for a review, see Johnson and Ait-Daoud 2000). SSRIs augment central serotonergic function and, by tonic inhibition, decrease mesocorticolimbic dopamine (DA) release. DA activation mediates alcohol's rewarding effects; hence, its diminution should be associated with decreased abuse liability. Moreover, in humans, there is solid evidence that individuals with the highest biological predisposition to alcoholism, typically by having an early disease onset, family history, or both, have reduced serotonergic function (Buydens-Branchey et al. 1989; Fils-Aime et al 1996; LeMarquand et al 1994a; LeMarquand et al 1994b; Swann et al 1999). It was, therefore, tempting to predict that alcoholics would benefit from SSRI treatment, and that those with an early onset and/or family history would benefit the most because the SSRI would presumably ameliorate the existing disequilibrium in serotonergic function.
Despite the encouraging results of earlier studies, more rigorous, well controlled, state-of-the-art trials have generally failed to find a therapeutic effect for SSRIs in treating alcoholism (Gorelick and Paredes 1992; Kranzler et al 1996).
In humans, functional control of the serotonergic system also seems to be regulated by genetic differences in SERT expression (Meltzer and Arora 1988). The SERT possesses the only known functional polymorphism regulating the serotonin system (Heils et al 1997; Heils et al 1996; Lesch et al 1997). Basically, the polymorphism of the SERT 5′ regulatory promoter region (5′-HTTLPR) on chromosome 17p12 consists of two types (Heils et al 1997; Heils et al 1996; Lesch et al 1997). The long (LL) variant, compared with the short (SS) or heterozygous (SL) form, is associated with three times greater 5-HT uptake from platelets (Greenberg et al 1999) and in lymphoblasts (Lesch et al 1996). Hence, individuals with the LL variant of 5′-HTTLPR can be expected to have increased SERT number and function and reduced levels of intrasynaptic 5-HT.
Recent scientific evidence would support the hypothesis of LL variant of 5′-HTTLPR predominance among EOA (Ishiguro et al 1999; Schuckit et al 1999). Turker et al. (1998) suggested that high ethanol tolerance may be associated with the SS/SL form of 5′-HTTLPR, but their rather informal criteria and the use of controls from a blood bank with uncertain alcohol histories may make their conclusions difficult to substantiate. Furthermore, a study by Sander et al. (1998) did not find a significant relationship (p=0.09) between SS/SL genotype and alcoholics with dissocial personality disorder. Finally, there are conflicting data on the relationship between the SS/SL form of 5′-HTTLPR and alcoholism in general (Edenberg et al. 1998; Hammoumi et al. 1999; Jorm et al. 1998; Sander et al. 1997); however, these studies contain no subtyping information. Moreover, it is difficult to compare these epidemiologic genotyping studies because of differing diagnostic criteria between the studies and different population frequencies across ethnic groups for the allelic forms. Perhaps most importantly, none of these studies have taken into account that it may be the interaction between these subtypes and alcohol consumption which is critical. That is, even though these allelic forms of the SERT may not determine vulnerability to alcoholism per se, the interaction between the allelic forms and alcohol consumption may determine treatment response, particularly to a selective serotonergic agent.
Reduced 5-HT neurotransmission has been reported in those with an increased propensity for drinking and in alcoholics who exhibit antisocial behaviors (i.e., EOA) (LeMarquand et al 1994a; LeMarquand et al 1994b). These results are consistent with: 1) the demonstration of increased 5-HT uptake into presynaptic serotonergic neurons in the brain, in lymphocytes, and in platelets of alcoholics and their descendants (Boismare et al 1987; Ernouf et al 1993; Faraj et al 1997) and 2) SPECT studies in nonhuman primates that had undergone early environmental stress, showing that increased binding of serotonin transporters is associated with greater aggressiveness and reduced sensitivity to ethanol intoxication (Heinz et al 1998). It would, therefore, be tempting to speculate that this hypo-serotonergic state could render individuals more vulnerable to experimentation with alcohol early in life.
Although acute alcohol intake may initially produce some temporary relief by increasing brain 5-HT levels, the residual effect is to reduce serotonin function, thereby setting up a vicious cycle (for a review see LeMarquand et al. (LeMarquand et al 1994a; LeMarquand et al 1994b)). Chronic excessive drinking does not result in sustained increases in 5-HT neurotransmission (Branchey et al 1981; Ledig et al 1982; Pohorecky et al 1978). Reduced SERT density in the raphe nuclei is associated with an early alcoholism onset in violent offenders (Tiihonen et al 1997) and with the combination of having the LL variant of 5′-HTTLPR and chronic drinking in both postmortem brains (Little et al 1998) and living individuals (Heinz et al 2000). The study of Heinz and colleagues (Heinz et al 2000) showed that individuals with the LL form of 5′-HTTLPR are more vulnerable to chronic alcohol-induced reductions in SERT density, but their study requires validation in an adequately powered prospective study that contains an equal number of individuals with the LL and SS/SL variants of 5′-HTTLPR. This would enable confirmation of the differential phenotypic expression of these allelic forms. Although it may, at first, seem paradoxical (i.e., for those with the LL variant of 5′-HTTLPR to have both reduced SERT density and decreased serotonergic function), it is notable that the SERTs in the raphe are associated with the regulation of cell firing rates.
There is a long felt need in the art for compositions and methods useful for diagnosing, treating, and monitoring alcohol disorders and susceptibility to alcohol disorders. The present invention satisfies these needs.